Full overnight closures are planned this weekend on the westbound Pomona (60) Freeway in Diamond Bar as crews work to improve traffic flow and safety at an interchange called one of country’s most congested. The closure between Lemon Avenue/Brea Canyon Road and Diamond Bar Boulevard is scheduled for 12:01 a.m. to 6 a.m. Saturday and 12:01 a.m. to 8 a.m. Sunday. Planned closures of ramps, connectors and streets are as follows: — The northbound Or…